Alternator Ground Feedback Loop?

[TX82FIAT]

Folks, I have an amp in the trunk to the right of the center battery box. Because the amp is so close to the battery (4 inches) I figured what better place to ground the amp then the negative terminal on the battery. I've always had a slight hum or ground feedback loop because the stereo head unit in the dash is grounded at the dash and the head is grounded to the battery. I figured the different resistance in the frame from the head unit was causing the alternator buzz.

About a month ago I put a new heater core in (works great) tore up the old tar off the floors removing rust, treating metal and POR15. I then insulated with sound deadener foil and put new carpets in. The odd thing is that the alternator feedback increased. Traced the lines and found no shorts to ground or poorly insulated crossover points. So, given we have a battery in the trunk and a head unit in the front. What benefit if any do you think there would be in establishing a better ground that is run from the head unit under the center console and up to the trunk. This would be about 7 feet of wire to the common negative post on the battery from the head unit ground wire.

I know all the stereo install guides say to install the grounds within 18 inches of the unit. It just seems to make sence that a common ground point would eliminate a ground loop. Or... should I just creat a new clean ground pooint under the dash to get the ground away from any interference at the existing star ground under the dash. Can not figure why a slight hum would escalate to an annoying wine given the scope of work done?

[rodman]

Take a 14 or so guage wire, strip it, and wrap it around the Line-Outs on your HU. Take out two screws on the back
casing, and wrap the wires around the screws. Tighten the screws. It should look like this:

[TX82FIAT]

That is an interesting concept. I think the RCA cable outer case is the ground for the signal wire. That makes the RCA wire grounded to the headunit that is grounded to the common ground point. I have the type of RCA outputs that are on a wires coming out of the head unit and not attached to the back of the unit, so I think i can solder a ground from the RCA connection to the head unit and accomplish the same thing.

I'll give this a try and see if it works as it can not hurt. However, I'm still a little foggy with respect to how this balances out resistance in the chasis between a ground up front with the head unit and a ground in the back with the amp that i suspect is causing the ground loop. Maybe it creates a ground with less resistance to the back of the vehicle through the RCA lines and the AMP. Or maybe it compensates for a poor ground in the signal wire that eliminates the noise. I don't know? I'll give it a try as I understand the concepts but struggle to find a solution to make it all work again.

I love the sound of the car. Part of the fun of this car is playing classic rock loud with either of my sons on a weekend cruze.

[vandor]

Maybe this would accomplish the same thing, but I was going to suggest to run a wire between the amps and the head units' grounding points.

[spider2081]

Have you tried a ground isolator? I have used products like this with reasonable results.

Metra Electronics METRA GL15 Ground Loop Isolator - Car Amplifier
http://www.amazon.com/Raptor-GL15-Groun ... B0009O19LQ

[TX82FIAT]

I have a 4 channel ground isolator on order $13. i hear these work very well. the only drawback being a drop off in the signal frequencies that can hurt sound quality. I'm going to try to fix the problem before patching or isolating the problem. However, a fix is a fix and I'm not going to be playing the stereo at crush volume to shake nuts and bolts loose or reverberate body panels like some kids do.

Anything is better than an alternator wine.

So, to sum up I may do one or all three of the following.

-Add a ground wire from the head unit to the amp ground mount position on the negative terminal of the battery with a fuse.

-Add a four small fused grounds from the RCA cable connection point to near the head unit to the screws on back of head unit itself.

-Add a four channel ground isolator.

this may be overkill but the combination should correct any and all ground noise. Verified RCA cables are good by taking out and using on stereo in house as well as computer to TV sound transmission. So... Unles something is wrong in the head unit this should work.

[aj81spider]

I'm not a stereo expert, but here are a couple of thoughts:

> Does the hum happen when the car is not running? If it does then it is not from the alternator.

> Unless your ground connection under the dash is poor, running a ground wire back to the battery doesn't seem like it will be better. The resistance of the car body should be less than the resistance of a wire running back to the battery.

> Did you move any of the wires (Speakers, grounds, power, etc) when you put in the floor? If it is crosstalk, rather than a ground loop you may have moved wires closer to the source of the problem - thus made the noise worse.

> If you really think it is a ground loop (and I'm not convinced of that) you could move the head to the trunk temporarily and ground it with a short wire to the battery. Then run jumpers to the speaker connections under the dash and see if the noise goes away. If it doesn't then it isn't a ground loop, and it is being introduced somewhere else.

Good luck - these types of problems are irritating and frequently hard to find.

[TX82FIAT]

AJ, that is some sound advice.

The hum only exists when the engine is on and it corresponses to RMPs.

I was thinking about the ground and resitance through the chasis compared to a wire. What I think I will do is use a 10 gauge copper fused wire down the center of the car from the head unit to the negative post on the battery. This will have very little resistance and results in a shared grounding point.

I did move almost all wires when doing the floor. I'm going to go back in this weekend to review and make sure circuits are physically isolated. Crosstalk is a possability. In fact, now that you have mentioned it. The floor is now insulated with an aluminum coated barrier. I will use some wire loom to isolate and insolate the wires so they are not in contact with the aluminum barrier at any point. i guess the cross talk idea scares me more than a gropunding issue because i could have some hot spot (emmitance/capacitance) almost anywhere that has been magnified by the aluminum barrier.

[TX82FIAT]

Solved, I did a few resistance and voltage drop calculations using tools on the intranet. I soldered the 16 gauge radio ground with a second ground on the back of the radio chassis in a fork to a 10 gauge wire. In installed a fuse at the juntion and ran the 10 gauge back to the battery negative terminal where the amp is grounded.

Ground loop eliminated and great sound out of all four speakers. Unconventiona,l but I figured the 10 gauge reduced resistance compensated for the 13 ft wire to the back of the car.